Automated loading and unloading of work pieces in and out of a processing apparatus and in between processing apparatuses are often accomplished with robotically controlled load and unload mechanisms. Particularly, wet semiconductor manufacturing processes have special requirements because the work pieces being transferred are often wet, may not allow contact except for at the periphery of the work piece, and may also require continued application of wet chemicals.
One such a manufacturing process is chemical mechanical planarization (CMP) of a wafer. The CMP method typically requires the work piece to be loaded into and mounted precisely on a carrier head in a manner such that the surface to be planarized is exposed. The exposed side of the work piece is then held against a polishing pad and polished using a relative motion between the work piece surface and the polishing pad in the presence of a polishing slurry. Following the planarization, each wafer is unloaded from the carrier head and transferred to a subsequent processing apparatus such as another polishing station, a buffing station, a cleaning station or a spin-rinse-dry (SRD) station.
The conventional CMP carrier head includes a flexible diaphragm against which the back surface (the surface that is not to be polished) is pressed. The flexible diaphragm is surrounded by an annular wear ring or retaining ring having an inner diameter only slightly greater than the diameter of the work piece to be polished. The diaphragm and the wear ring form a cavity into which the work piece must be loaded. To carry out the planarization operation, the work piece must be mounted against the diaphragm within the confines of the wear ring. In the CMP processing of a wafer the recess into which the wafer must be loaded has a depth on the order of the thickness of the wafer itself, or about 0.75 mm, and the clearance between the inner diameter of the wear ring and the outer diameter of the wafer is typically less than 1 mm.
A CMP load cup mechanism capable of transferring a work piece to the carrier head such that the work piece can be mounted within the confines of the wear ring with a high degree of precision and without contacting the front surfaces of the work piece was described in U.S. patent application Ser. No. 10/821,758 titled “CMP Apparatus and Load Cup Mechanism” and filed Apr. 9, 2004 and U.S. patent application Ser. No. 10/884,371 titled “CMP Apparatus and Method” filed on Jul. 2, 2004, both of which are incorporated herein by reference in its entirely for all purposes. With this load cup mechanism, a front-end robot first transfers a work piece, such as a semiconductor wafer, from a work piece cache to a load or handoff station from where a back-end robot picks up the work piece. The back-end robot moves the work piece to one of several load cups, each associated with a process station, also known as a platen. The load cups receive the work piece and moves the work piece to a position directly beneath a carrier head. There, the carrier head and the load cup engage, and the work piece is transferred to the carrier head. The now empty load cup moves away from the process station, and the processing ensues. After the work piece is planarized, the reverse happens. The load cup engages with the carrier head to receive the processed work piece, and moves the work piece to a “home” position where the back-end robot picks up the work piece. The back-end robot then loads another work piece onto the load cup, which transfers it to the carrier head associated with that load cup.
However, from the time the load cup mechanism unloads the work piece from the processing station to the time the load cup mechanism returns to the station with an unprocessed work piece, the station sits idle. Because one backend robot may serve four or more process stations, this loading and unload of work pieces onto the load cups can become a bottleneck. If more than one station requires a new work piece, the load cup mechanism would have to wait for the robot to become available. While a load cup waits for the back-end robot, the process station sits idle. It is therefore desirable to provide an improved work piece handling mechanism that reduces the idle time and increases the throughput of the tool.